The main purpose of the present work was to investigate the influence of urea
concentration on hydroxyapatite (HAp) synthesis. Calcium nitrate and ammoniumdihydrogenphosphate
were used as Ca and P sources. A homogeneous precipitation agent – urea -
was applied in the field of ultrasonic irradiation. Different Ca : P : urea molar ratios were applied
and the samples were characterized by X-ray diffraction (XRD), scanning electron microscopy
(SEM) and infrared spectroscopy (IR). The results reveal the influence of the urea concentration on
the phase composition of the system, crystallite size and the particle morphology. Different Ca : P :
urea molar ratios led to various system formation: from biphasic OCP and HAp to monophasic
HAp. The estimated HAp crystallite size was 24.4 nm. An explanation of the mechanism of the
HAp synthesis is given, regarding the formation of the intermediate and final product during
ultrasonic irradiation.